1. Field of the Invention
The present invention relates to a magnetic bearing device, and a vacuum pump having the magnetic bearing device.
2. Description of the Related Art
In a device such as a magnetic bearing type turbo-molecular pump in which a rotor is supported by a magnetic bearing device in a non-contact manner, magnetic attraction of an electromagnet (a current of the electromagnet) is feedback-controlled in real time based on a deviation (displacement) between a levitation position and a target position of the rotor in order to maintain a levitating state of the rotor on a predetermined target position.
A system in which displacement is detected by a dedicated displacement sensor is used mainly. In recent years, due to compactification, price reduction and reliability improvement, a sensorless (self sensing) device in which a dedicated sensor is omitted and an electromagnet for generating a levitation control force is used as not only a conventional actuator function but also a sensing function (inductance system) is being put to practical use.
In the inductance system, a high frequency carrier wave (sensor carrier) is applied to a dedicated sensor or an electromagnet coil, and an amplitude of the sensor carrier is modulated due to an inductance change caused by a levitation gap and is remodulated. As a result, a levitation gap signal (displacement signal) is obtained. In the demodulating process, a lot of systems in which an AD converter synchronously samples a modulation wave signal by applying a digital technique so as to be taken in, namely, direct systems in which a smoothing process that causes a delay is not necessary are proposed. Known direct type sensing is disclosed in, for example, JP 2006-308074 A, JP 2000-60169 A, and JP 2001-177919 A.
The technique disclosed in JP 2006-308074 A has a dedicated sensor, and a relationship between a sensor carrier frequency fc and a sampling frequency fs at a time of sampling a modulation wave signal is such that fs=2fc or fs=fc/n (n is a natural number). Since only a sensor carrier signal voltage is applied to the dedicated sensor, normally S/N of a signal is satisfactory. However, like a case where a device including a magnetic bearing is arranged so that the electromagnet and the dedicated sensor are extremely close to each other in order to realize compactification, when a magnetic flux caused by a control current for exciting the electromagnet exercises an effect on a signal of a dedicated sensor coil, a control current component (noise component) might be mixed in a signal component modulated by rotor displacement due to the effect of the magnetic flux.
For this reason, normally a band-pass filter (the band-pass filter mainly for the sensor carrier frequency fc) provided right in front of the AD converter filtrates the most part. However, in order to completely remove a noise component, a Q value of the band-pass filter is further increased and a band should be narrowed. However, when the band of the band-pass filter is narrowed, the modulated displacement signal is greatly delayed from an original signal. As a result, the magnetic bearing control itself is deteriorated, and its application is limited. For this reason, a noise component remains in an input signal of the AD converter, and a noise effect is exercised on the demodulated signal. Therefore, a vibration component that is not actually displaced (vibrated) is mixed in the demodulated rotor displacement signal, and displacement information is directly fed back and levitation control is made. As a result, the rotor is forcibly vibrated by the noise component, and its reaction force is transmitted to the stator side and occasionally causes a device vibration.
The technique disclosed in JP 2000-60169 A relates to both the dedicated sensor and sensorless devices. In devices having the dedicated sensor, a square wave signal obtained by inverting a code at every sampling time under a condition that fs=2fc is generated by a digital process and is output from the DA converter, and the output signal is modulated as a sensor carrier signal by the displacement signal (rotor displacement) in a sensor. A modulated wave of the signal is synchronized with a peak timing at the same frequency fs (=2fc) so as to be fetched. In the demodulating process, since the signal data fetched by the AD converter is processed by inverting a code at everyone sampling (a code is inverted at a minimum peak time of the sensor carrier), similarly to the case of the invention described in JP 2006-308074 A, a problem of vibration generation arises.
Further, in the case of the sensorless type, the sensor carrier signal is superimposed on an electromagnet driving current signal and the superimposed signal is output from the DA converter so as to excite the electromagnet via a power amplifier. An amplitude of the superimposed sensor carrier signal is modulated by an electromagnet coil. For this reason, an amplitude modulation signal including the displacement signal component is extracted, and the modulating process that synchronizes with the sensor carrier is executed similarly to the case with the dedicated sensor. However, in the case of the sensorless type, since the displacement signal is sensed by the electromagnet instead of the dedicated sensor, not only the modulation signal of the sensor carrier signal to be superimposed but also a control current signal is mixed at an equivalent or more signal level. Therefore, a control current component (noise component) to be mixed in the amplitude modulation signal is increased more than that in the case of the dedicated sensor type.
The technique described in JP 2001-177919 A relates to the sensorless type. A sensor carrier component for sensing is superimposed on a driving current for exciting an electromagnet. The basic signal process is similar to that described in JP 2000-60169 A, but they are different in the following point. That is to say, the sensor carrier (carrier wave) to be superimposed on each of paired electromagnets opposed to each other across the rotor is applied in an opposite-phase relationship. As a result, an amplitude modulation signal including a displacement signal component is efficiently separated so as to be capable of being extracted from a control current component. However, due to the similar reason to the case of the sensorless type in JP 2000-60169 A, a problem that a noise is mixed in a displacement modulation signal arises.